Chromatin structure of the chicken lysozyme gene domain as determined by chromatin fractionation and micrococcal nuclease digestion

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Abstract
The chromatin structure encompassing the lysozyme gene domain in hen oviduct nuclei was studied by measuring the partitioning of coding and flanking sequences during chromatin fractionation and by analyzing the nucleosome repeat in response to micrococcal nuclease digestion. Following micrococcal nuclease digestion, nuclei were sedimented to obtain a chromatin fraction released during digestion (S1) and then lysed in tris(hydroxymethyl)aminomethane-(ethylenedinitrilo)tetraacetic acid-[ethylenebis(oxy-ethylenenitrilo)]tetraacetic acid and centrifuged again to yield a second solubilized chromatin fraction (S2) and a pelleted fraction (P2). By dot-blot hybridization with 14 specific probes, it is found that the fractionation procedure define three classes of sequences within the lysozyme gene domain. The coding sequences, which partition with fraction P2, are flanked by class I flanking sequences, which partition with fractions S1 and P2 and which extend over 11 kilobases (kb) on the 5'' side and probably over about 4 kb on the 3'' side. The partitioning of class II flanking sequences, which are located distal of class I flanking sequences, is different from that of class I flanking sequences. Coding sequencs lack a canonical nucleosome repeat, class I flanking sequences possess a disturbed nucleosome repeat, and class II flanking sequences generate an extended nucleosomal ladder. Coding and class I flanking sequences are more rapidly digested by micrococcal nuclease than class II flanking sequences and the inactive .beta.A-globin gene. In hen liver, where the lysozyme gene is inactive, coding and class I flanking sequences fractionate into fractions S2 and P2. Chromatin fractionation of steroid-induced and deinduced chick oviduct nuclei shows a close correlation between active transcription of the lysozyme gene and enrichment of coding sequences in fraction P2. Our results indicate that the partitioning of coding lysozyme gene sequences with low-salt insoluble nuclear material relates to the transcriptional process along these sequences.